itunes is not the product that would get the NSA salivating - not much of it's data was that useful imo before the phones came out. It's about phones and messaging. When did imessage get released? How long have apple been able to track your location with GPS? How long have you been storing your photos on the cloud (whether or not you realised it) etc etc.
Once they had 1-2 of the big tech companies as precedent they really just would have started to go round to anyone they thought might be of interest. Because that is the whole point. Collect everything you can get your grubby mitts on and sort through it later.
If the NSA actually pays any consideration to how the value of their data helps find "terrorists"/criminals, iTunes data is probably not particularly valuable.
